That's why i said it's massive headcanon. I don't talk about Kidd cos we don't have exact intel of his strength as he was off-screen the whole time until the raid. But for Law we have a whole arc dedicated to him, how much he stack up to Doffy, and we got the answer, Doffy at worst mid diff him. By the end of Dressrosa, Luffy>> Law.
You think Law off-screen training grew 3 times stronger to fact that he could be stronger than Luffy that went hard battle in WCI, facing an opponent that is even stronger than Doffy that whopped Law and eventually came out on top.
What off-screen training your referring to?? Law and the others keep hiding and doesn't cost any ruckus in Wano until Luffy came. Luffy grew in strength when he battle someone initially stronger than him(Katakuri). What did law train? by himself?? Kinemon teach him?? Or kiki?? who else?? If you're gonna get substantially stronger in those short time you need to train with someone stronger than you, someone who can teach you new teachnique. Law has none, he was with Kinemon, his crew and other SH. Lol